Add the missing input check for "diff" field of "ledger" (#643)

Fixes #644

src/rpc/handlers/Ledger.h

@@ -77,6 +77,7 @@ public:
             {JS(transactions), validation::Type<bool>{}},
             {JS(expand), validation::Type<bool>{}},
             {JS(binary), validation::Type<bool>{}},
+            {"diff", validation::Type<bool>{}},
         };
 
         return rpcSpec;

unittests/rpc/handlers/LedgerTest.cpp

@@ -152,6 +152,12 @@ generateTestValuesForParametersTest()
             "invalidParams",
             "Invalid parameters.",
         },
+        {
+            "DiffNotBool",
+            R"({"diff": "x"})",
+            "invalidParams",
+            "Invalid parameters.",
+        },
     };
 }